Simple DC power supply with overcurrent protection design

At present, a variety of DC power supply products are now flooding the market, power technology has been more mature. However, based on cost considerations, performance requirements for power is not high occasions, can be integrated with the regulator over-current protection circuit, the same products to meet the requirements. Over-current protection circuit in power supply circuit as an indispensable component of the control method in accordance with its turn-off can generally be divided into current limiting mode and manner, and the DC motor power more suitable way off.

Over-current protection circuit for a current sampling the first link in common is a small resistor in series or a Hall element to obtain current signal. Hall element as a result of relatively large size, which is very expensive, so consider using a small resistance in series approach.

1 Working Principle

With over-current protection circuit of the LM317 voltage regulator as shown in Figure 1, integrated circuit voltage regulator will generally be divided into 5 parts, namely the exchange of step-down circuit, rectifier, filter circuits, regulator circuits, protection circuits. 220V AC power supply voltage by the step-down transformer rectifier DC voltage to be Vin, the voltage input through the filter circuit to the integrated input voltage regulator in the integrated voltage regulator output can be 1.25 ~ 37V DC voltage. Part of the work and the schematic voltage waveform as shown in Figure 2.

The following analysis of the working process of the protection circuit.

1.1 Integration of the protection of regulators

In order to obtain higher output voltage value, LM317 voltage regulator adjustment between the client and the value of the resistor R2 and the pressure drop is often greater in R2 than at both ends and then a 10μF capacitor C3, to be effective in inhibiting output ripple. When the input or output short-circuit when the discharge capacitor C3 will have an impact on the R1 voltage, will endanger the voltage regulator circuit, so need to R1 and diode D3 at both ends to protect the regulators.

Regulator output capacitor can not work, as regulators in the 1:1 depth to work under negative feedback, when the output of a capacitive load value, the regulator may appear the phenomenon of self-excited . Therefore, the regulator access to 0.1μF input capacitor C1, output access 1000μF electrolytic capacitor C5, the current provision of adequate supply, at the same time to prevent possible self-excited oscillation, as well as reduce noise and improve high-frequency load transient response. When input occurs when short-circuit, C5, through regulator to adjust the discharge pipe, C5 greater value, then the impact of discharge a large current and voltage through an internal voltage regulator output transistor discharge, which may result in the output transistors to launch anti-node to breakdown. To this end, the regulators at both ends and then diode D2, input short-circuit discharge C5 through D2, the protection of regulators.

1.2 over-current protection

Over-current protection circuit see Figure 3, R5 for sampling small resistance. When the power supply when the output voltage regulator positive DC output voltage, electric start. DC motor to start due to the instantaneous current iout larger (approximately rated current of 8 to 10 times), iout flow is too small resistor R5, and R4 by the charge on the C4. By setting the R4, C4 value, so that the charging time is greater than Υ motor start-up time δ, V2 (9013) in a closed state to the steady-state electric start, the current return to the operating current. Once the electrical short circuit or block transfer, so that C4 capacitor voltage to reach the turn-on voltage V2, then V2 conduction, forced output voltage regulator reference voltage dropped to 1.25V.

2 Circuit Design

2.1 Selection of integrated voltage regulator

Regulators in the choice of integration should take into account the performance of several aspects of the use and price. Performance indicators are based mainly on the size of the load voltage and current, adjusting the rate and scope of the work of the width and stability to the election. LM317 Series because of its adjustable output voltage, at the same time the regulator has a high accuracy, high ripple rejection ratio and better temperature characteristics of output voltage, which is widely used.

For total power output power P0, load rated voltage U0, the output current rating for I0 = P0/U0, in order to enable stable operation of the circuit, but also need to consider a certain design margin (more than 10% of the general admission). LM317 series voltage regulator as the main parameters listed in Table 1, according to the calculated current value, select the appropriate regulators.

Choice based on diode reverse voltage diode current VRM and positive IF. As a result of the greater capacity of filter capacitance, diode conduction angle smaller pulse current through the diode the greater the magnitude, the amplitude of the rectifier current must be taken into account. The average flow through rectifier current ID = Ii / 2, Ii = IR2 + I0, IR2 = IR1 + Iadj ≈ 0.01A (where Ii is the input current regulator, IR1, IR2, Iadj were flowing through R1, R2, as well as the adjustment of the current client), the ID = (0.01 + I0) / 2. Taking into account the impact of capacitive charge current, positive current general admission average current of 2 ~ 3 times.

Maximum diode reverse voltage , In the U2-type power transformer secondary voltage for the RMS, Ui for the rectifier output voltage (that is, the input voltage regulator). LM317 voltage regulator in order to ensure stable operation of the input voltage Ui and the difference between the output voltage U0 general range of 5 ~ 15V in, check Ui-U0 = 1OV, a Udmax = 1.2Ui = 1.2 (U0 +10) = 12 +1.2 U0. Designed to be taken into account a certain margin.

Regulator circuit in tandem to determine the transformer secondary voltage is very important. In order to have a surplus and if the secondary voltage has a higher adjustment will increase the loss of control, so that the radiator was increased accordingly. Therefore, in order to design a performance of power transformer parameter values always have to go through several adjustments. Reference to the history presented in a comprehensive manner on a power transformer noise design elements, this article will not repeat them. This approximate calculation method used to determine the U2 and I2.

U2 = Ui/1.2 = 0.83 (10 + Uo)

I2 = (1.5 ~ 2) Ii = (1.5 ~ 2) × (0.01 + Io)

2.4 voltage regulator integrated circuit design

In order to ensure the no-load voltage regulator can work properly. Is flowing through resistor R1 of the current can not be too small. General admission IR1 = 5 ~ 10mA, so R1 = VREF/IR1 = 1.25 / (5 ~ 10) × 10-3 ≈ 120 ~ 240Ω, where VREF for reference voltage regulator. U0 and the output voltage VREF, R1, R2 have the following relations:

Un = VREF + (IR1 + Iadj) R2 = (1 + R2/R1) VREF + IadjR2 (1)

Adjust resistor R2, you can change the size of the output voltage. Iadj as a result of a very small (only 50μA), so type (1) can be written as Uo = (1 + R2/R1) VREF = 1.25 (1 + R2/R1) (2)

By Type (2) obtain R2 = (0.8U0-1) R1.

2.5 Protection Circuit Design

Diode protection circuit is relatively simple choice, as long as they can guarantee to meet the impact of reverse voltage and current requirements of these two it. The main role of R3 is to limit the transistor's base-level current, generally take 1 ~ 2kΩ. Here to talk about the over-current protection circuit design.

2.5.1 Starting

Motor starts charging time must meet the start-up time greater than Υ δ, V2 does not turn-on, start the motor to work properly. Starting current due to a large, generally rated current of 4 to 7 times, can be seen as the same, as I = 5I0. According to Figure 4, we can see.

2.5.2 the protection of state

Based electric load operation at rated conditions, the electrical current has stabilized I0. Electrical short circuit or stall, the current suddenly increased to short-circuit current Is, the beginning of charging capacitor C4. Consider a certain design margin, take the protection of current settings IG <IS, type (3) the same as the first order to set up a full response equation, initial conditions uc4 (0 +) = I0R5, a mandatory component uc4 (∞) = IGR5 , to solve a

2.6 Thermal Design

Regulators of the maximum allowable power dissipation depends on the maximum junction temperature of chip TjM, when T <TjM when the voltage regulator to work properly. Therefore, the regulator of the heat capacity of the stronger, the lower junction temperature, it can bear the greater power also. Regulators depend on the cooling capacity of its semiconductor devices increases resistance to heat sink can reduce the total thermal resistance. Rθ1 that if the device from the junction-to-case thermal impedance, Rθ2 said casing from the device to the surface of heat sink thermal resistance, RθA said heat sink from the node to the surface of the thermal resistance, the RθA = Rθ1 + Rθ2. Rθd that if the heat sink to ambient thermal resistance, Rθ 'said the increase in heat sinks into the air after the end of the total thermal resistance, the Rθ' = RθA + Rθd. Integrated voltage regulator for the maximum allowable junction temperature for TjM, the maximum ambient temperature for TAM, plus the radiator after the device power dissipation is PD, there are relationship (9).

Rθd obtained through inspection of equivalent heat sink thermal resistance and the relationship between material thickness and surface area of the manual. The scope of available surface area. Table 2 lists several commonly used form of thermal resistance package.

According to Table 3 to select the design parameters of the assembly after the determination by experiment to achieve the following technical indicators:

Output characteristics Un = 24V, In = 0 ~ 1A:

Voltage stability Sv ≤ 5 × 10-6;

Load stability of S1 ≤ 5 × 1O-5;

Temperature coefficient α ≤ 1 × 10-5 / ℃.

Motor can be started, when the motor stall, the normal role of over-current protection.

4 Conclusion

To sum up, the use of LM317 integrated voltage regulator designed to power low-power DC motor, the circuit structure is simple, efficient, low cost, good performance of the output voltage has a very good application prospects.